1. Field of the invention
The present invention relates to a superconducting thin film possessing a high critical temperature, and more particularly, it relates to a superconducting thin film improved in the critical current density.
2. Description of the related art
The superconductivity is a phenomenon which is explained as a phenomenon of phase change of electrons in which the electrical resistance become zero and the perfect diamagnetism is observed.
A variety of superconducting devices have been proposed and studied in the field of electronics. One of typical applications of the superconductor is a device using the Josephson effect in which quantum efficiency is observed macroscopically when electric current is passed through a weak junction arranged between two superconducting bodies. Tunnel junction type Josephson device which is another typical application of the Josephson effect is expected to be a solution for realizing a highi-speed and low-power consuming switching device owing to a very narrow energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a quantum phenomenon precisely. Development of the superconducting devices is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption. It is also expected to utilize the superconductor in wiring material for electric circuits to reduce loss of power.
However, the critical temperature of superconductors could not exceed 23.2K of Nb.sub.3 Ge which was the the highest Tc for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64(1986) 189]
It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These superconductors, however, possess a rather lower transition temperature of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity.
This new oxide type superconducting material discovered by Bednorz and Muller is [La, Ba].sub.2 CuO.sub.4 which is called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to a known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such a high Tc as 30K which is extremely higher than the known superconducting materials.
It was also reported in the news parer that C. W. Chu et al. discovered in the United States of America another type of superconducting material having the critical temperature of about 90K in February 1987. This superconductor called as YBCO is a compound oxide of Ba-Y system represented by YBa.sub.2 Cu.sub.3 O.sub.7-.delta..
Recently, it was reported that still another type compound oxides possessing superconducting property were discovered. These now superconductors are Bi-Sr-Ca-Cu type and Tl-Ea-Ca-Cu type compound oxides and possess Tcs of higher than 100K and improved chemical stability so that they do not suffer from deterioration during usage.
Thus, from the discovery of the abovementioned new type of compound oxide superconductors, the realization of high-temperature superconductors have arisen suddenly.
The thin film of superconducting material has been produced by the physical vapour deposition technique such as a sputtering method in which a sintered oxide is sputtered with Ar gas to deposit the resulting evaporated particles on a substrate. Then, the deposited thin film is heat-treated in an oxygen containing atmosphere or is exposed to an oxygen plasma.
The physical vapour deposition technique has been used for producing a thin film of the conventional type superconductors such as Nb.sub.3 Ge and BaPb.sub.1-x Bi.sub.x O.sub.3. In case of a thin film of Nb.sub.3 Ge, particles of Nb and Ge are sputtered out of several targets each consisting of Nb and Ge respectively and are deposited onto a substrate to form a film composed of Nb.sub.3 Ge. Japanese patent laid-open No. 56-109,824 discloses a process for producing a thin film of A compound oxide represented by the formula: BaPb.sub.1-x Bi.sub.x O.sub.3 by means of sputtering technique.
The superconducting property of the compound oxide type superconductors deposited on a substrate is very sensitive to the oxygen deficiency in its crystal structure, so that, if the oxygen deficiency is not within an optimum range, a high Tc can not be observed and the discrepancy between the onset temperature and the critical temperature can not be reduced. Still more, the thin film of the superconducting compound oxide is apt to deteriorate.
One of problems of the superconducting thin films of the new type compound oxide is that they do not possess high critical current density (Jc). Namely, the critical current density of thin films prepared heretofore showed only several hundred A/cm.sup.2 which is too low to utilize actually the thin films in electronics devices.
The present inventors have proposed processes for producing a thin film of compound oxide having a high Tc in U.S. patent application Ser. No. 154,714 titled "Process for preparing a superconducting thin film" filed in Feb. 5, 1988 and Ser. No. 167,895 titled "Process for preparing a superconducting thin film" filed in Mar. 14, 1988 in which a thin film is prepared by sputtering technique. The present inventors have continued to improve the superconducting property of the thin film and completed the present invention.
Therefore, an object of the present invention is to provide an improved superconducting thin film composed of compound oxide which possesses a higher critical current density (Jc) in addition to a high critical temperature (Tc).